Code Quality: Rename RootNamespace Backend with Core

[0x5bfa]

Code Quality: Rename RootNamespace Backend with Core

I appologise for creating such a big diff.

Motivation & Context

• Improved readability

Warning
Please do not review code lines that I did not changed. I cannot make more diff. If there're more codes that are preferred to change, I will do that and mark that PR as the final one for that stage.

PR Checklist

• Were these changes approved in an issue or discussion with the project maintainers? In order to prevent extra work, feature requests and changes to the codebase must be approved before the pull request will be reviewed. This prevents extra work for the contributors and maintainers.
  Related Feature: Improving Performance, optimizations, reliability and codebase  #4180
• Did you build the app and test your changes?
• Did you check for accessibility? You can use Accessibility Insights for this.
• Did you remove any strings from the en-us resource file?
  • Did you search the solution to see if the string is still being used?
• Did you implement any design changes to an existing feature?
  • Was this change approved?
• Are there any other steps that were used to validate these changes?

Screenshots

None

[yaira2]

Let's wait for #12601 to be merged.

[0x5bfa]

I don't think this will affect to it badly because that PR doesn't develop Backend logic parts.